4 Only one program ran at a time, with unrestricted EDSAC, early 50’seffective address=pysical memory addressOnly one program ran at a time, with unrestrictedaccess to entire machine (RAM + I/O devices)Addresses in a program depended upon wherethe program was to be loaded in memoryBut it was more convenient for programmers to writelocation-independent subroutines⇒ How could location independence be achieved?

7 What is an advantage of this separationWhat is an advantage of this separation? Used today on Cray vector supercomputersPermits sharing of program segments.

8 As users come and go, the storage is “fragmented”As users come and go, the storage is “fragmented”. Therefore, at some stage programs have to be movedaround to compact the storage.Called Burping the memory.

11 Special registers just for the current user: Space required by the page tables is proportional to the address space, number of users, ...⇒ Space requirement is large too expensive to keep in registersSpecial registers just for the current user:-What disadvantages does this have?may not be feasible for large page tablesMain memory:-needs one reference to retrieve the page base address and another to access the data word⇒ doubles number of memory references!Affects context-switching overhead, and needs new management instructions.

13 There were many applications whose datacould not fit in the main memory, e.g., PayrollPaged memory system reduced fragmentation but still required the whole program to be resident in the main memoryProgrammers moved the data back and forth from the secondary store by overlaying it repeatedly on the primary storetricky programming!